A non-linear optical effect is strikingly displayed when exposing a material to intensive light such as laser beams. Such effect can be applied to a frequency conversion, an intensity moderation, a switching operation, and so forth. In recent years, many researches and developments have been made for obtaining the materials capable of displaying such a non-linear optical effect so far. Regarding such research and development of the non-linear optical material, "Non-linear Optical Properties of Organic Molecules and Crystals, vol. 1, 2, edited by AT&T, Academic Press Inc. '87", can be referred.
For the frequency conversion and, in particular, for the Second Harmonic Generation (hereinafter abbreviated to SHG) originated from the second-order non-linear optical effect, there have been some indications of the possibility that an organic compound may display extraordinarily higher efficiency than those of the conventionally known inorganic materials such as lithium niobate (LiNbO.sub.3), potassium dihydrogenphosphate (KDP), and so forth. Such an indication is found out in, for example, "Organic Non-linear Optical Materials", compiled under the supervision of Masao Kato and Hachiro Nakanishi, published by C.M.C. Company, Japan, 1985.
The optical non-linearity of an organic compound originates from .pi. electrons of molecules, and the second-order non-linear molecular polarizability (.beta.) becomes particularly greater when the above-mentioned compound has both an electron donative group and an electron attractive group.
There are, however, many compounds including, typically, p-nitroaniline which display no SHG at all or a few in their crystalline arrangement, even if they have a great molecular non-linear polarizability. This is because the molecular orientation in the crystals of polar organic compounds are liable to be centrosymmetric in crystalline arrangement.